You might try a product called BESTINE. It's a rubber cement solvent, similar to Goof Off, but much, much better. It can be found at most art supply stores or even Home Depot, I think. It is very flammable, so be sure to use in a well ventilated area. It's also really hard on your hands, so wear gloves for large jobs.
It won't harm most (there's always an exception!) plastics, but try it in a small inconspicuous place first.
Once you have used this stuff you'll never be without it for all those pesky little price tags & such that seem to be impossible to remove. Good Luck...